The invention relates to a marine drive having two counter-rotating surface operating propellers.
The present invention arose during development efforts directed toward a marine drive enabling increased top end boat speed. This is achieved by raising the torpedo out of the water to reduce drag, and by using two counter-rotating surface operating propellers. Reducing torpedo drag by raising the torpedo above the surface of the water is known in the art, for example U.S. Pat. No. 4,871,334, column 3, lines 35+.
In one aspect of the invention, simple, effective retaining structure is provided for holding the propeller-bearing-supporting spool in the horizontal bore of the torpedo fixed for non-rotation in each of the opposite rotational directions of the counter-rotating propellers. This prevents rotational loosening of the spool in each of the rotational directions as the propellers strike and pierce the surface of the water.
In another aspect, structure is provided to minimize shaft wobble tendency as the propellers strike and pierce the surface of the water.
In another aspect, a thrust bearing is provided which transfers thrust from a hollow outer propeller shaft to an inner counter-rotating concentric propeller shaft. The thrust bearing is located between fore and aft driven gears on the propeller shafts. The propeller shafts float within their respective gears.
In another aspect, combinations of carbon steel and stainless steel are provided for the propeller shafts to afford bearing support and corrosion protection where needed.
In another aspect, propeller self-centering mounting structure is provided to maintain proper propeller mounting notwithstanding surface operation vibration as the propellers strike and pierce the surface of the water.